


Motivation

by freidacay



Category: Undertale (Video Game)
Genre: Bara!Sans, Big Sans, Blow Jobs, Cunnilingus, Desk Sex, Established Relationship, F/M, Partial Nudity, Sensitive bones, or at least... i think... i tried, tra la la beware the woman who is bad at tagging
Language: English
Status: Completed
Published: 2016-01-02
Updated: 2016-01-02
Packaged: 2018-05-09 20:11:33
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,864
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/5553731
Author URL: https://archiveofourown.org/users/freidacay/pseuds/freidacay
Summary: <blockquote class="userstuff">
              <p>When a bender in Alphys' basement lab has Sans spending less and less time with you, you decide to take matters into your own hands.</p>
            </blockquote>





	Motivation

**Author's Note:**

> IT BIG BOY!SANS. Sans is taller and wider than reader in this one, because so help me, I can't get over big!sans. ([also...](http://pyrogothnerd.tumblr.com/post/134986406158/height-of-the-monsters-proof-that-sans-isnt)) My thirst is eternal. I ~~don't want to be~~ cannot be saved.
> 
> Also @sanscrap I tried writing some sensitive bones for you, but it started getting really REALLY specific, and I started groaning at my own writing, so I scrapped some parts. I hope the hints of it here are what you were looking for. I'm sorry. I have Failed ;-;
> 
> Please tell me if I left any mistakes behind! Enjoy!

The first time Sans and Alphys disappear into Alphys' basement together, you don't give it much thought. They're back up in less than an hour, smelling strongly of chemicals and ash. The two of them gesticulate wildly as they discuss their shared experiment, throwing about lingo and large words that fly completely over your head.

You shoot Undyne a confused look, to which she shrugs and whisper-shouts, "They're NERDS!"

Then she pulls you into a headlock until you agree to play Mario Kart with her.

Later on, Sans gently warning you not to come too close has you a bit worried, but he bundles you into his side after he takes a quick shower, and watches as Undyne destroys you on Rainbow Road. The satisfied tilt of his smile is hopelessly endearing. If he's happy, you're fine with it.

* * *

It happens again when the two of you visit Undyne and Alphys for the holidays. The four of you sit down to enjoy a small dinner. Alphys and Sans immediately begin discussing things like resonance frequencies and relativity, and eventually become so worked up that Undyne forcibly ejects them from the living room. You and Undyne eat together while the two of them escape to the basement yet again. They're under for so long that you end up staying the night, not wanting to leave Sans behind. Undyne lends you some of her pyjamas to sleep in and they're so big on you that you feel not unlike a child, but you appreciate the gesture anyways.

Sans appears in the guest bedroom in the middle of the night, by way of the teleportation magic he thinks you don't know about. When he climbs into the bed with you, you roll into him. He curls an arm around your waist to pull you into his front, and his other hand slides into your pyjama pants. He works your clit until you cum with a small gasp, and then gently shushes you when you try to return the favor. His gaze on you is soft as you drift off to sleep.

The next morning, he and Alphys are in the lab again. Admittedly, you're a little worried.

* * *

From then on, it becomes a routine. Sans' excuses for visiting Undyne and Alphys vary from ridiculous to half-hearted. Eventually, he gives up on trying to come up with them altogether. On the days when you don't see him, he texts you a steady stream of atrocious puns, randomly affectionate thoughts, and worrying pictures of the various experiments that he and Alphys have done. Undyne texts you her grievances. ( _oh my god!!! this is worse than when sans and the queen were telling bad jokes together!!! oh wait, that's all the time so basically THIS IS REALLY BAD. Come get your boyfriend PUNK! I miss Alphys!!!)_

You're happy Sans has rediscovered his love for science. You remember him telling you that it had been ruined for him when someone he cared about was hurt in an accident. He doesn't really enjoy his work as a bouncer, even though it pays well. However, you understand Undyne's irritation. It's difficult enough to sync up your schedule with Sans' as it is. Unless something motivates him into action, he's extremely lazy about most everything. The thought that whatever it is he's working on could make him want to spend less time with you stings just a bit. You also feel that you're being a little ridiculous. It's probably not that serious.

Still, when Undyne sends you yet another frustrated text message, you decide to hatch a small plan. There's always ways to find the fun in any situation, right? Being with Sans has taught you that much a million times over.

Your reply to Undyne is cryptic. ( _You should get Alphys out of the lab tomorrow. I'll try and get Sans to come up for air.)_

* * *

The next day, Undyne drives up to the house you share with Sans and Papyrus. Alphys is in the passenger seat. You walk up to their car, and you can't help a wry smile at the sheepish look on her face.

"We kinda went overboard, huh?" Alphys asks.

"Yeah, you did!" Undyne shouts. Alphys jumps, and then swats at her arm with a tiny laugh.

You learned pretty early on that Undyne sounds angry a vast majority of the time, but the crooked smile on her face tells you she's just pleased to be able to get her girlfriend out of the lab.

Alphys reaches into her pocket, and after a moment of rummaging, she hands you two small keys.

"Here," She says. "Blue for the house, red for the lab. Sans should be at his desk organizing data. When you go in the basement, just go down the hall and the last door on the right is Sans'. I'm sorry about all this."

You are quick to reply. "Don't be sorry! I'm glad you and Sans are having fun. I just miss seeing him more often, is all. Anyways, where are you guys going?"

"We're going to the beach!" Undyne informs you, excitedly slamming her hands into the steering wheel.

Alphys grins. "Let's try not to break anything this time."

"You guys have fun!" You say, backing away from the car.

"I hope you can get bone-boy out of the lab! Oh, and nice outfit, punk!" Undyne shouts as she drives off. You blush as you wave goodbye. You're hoping Sans will like it as well.

The drive to Undyne and Alphys' house isn't long—when they'd first come to the surface, Sans' circle of friends had made the unanimous decision to live close if they could manage it—but it still feels like forever. When you arrive, you rush through the house. One quick twist of the red key, and the basement lab is accessible to you for the first time. You are met with a flight of stairs. You take them two at a time.

It's eerily quiet. The lighting of the long hallway with its rows of unmarked doors, along with the sterile scent that permeates the the air, makes you feel markedly alone. Before long, you're speed-walking down the hall. You can't shake the feeling that there are eyes boring into your back. When you finally reach Sans' door, your knocks are a little frantic.

"Alphys? Thought you and Undyne were goin' out or something." Sans calls, his voice gravelly from obvious disuse.

"It's me!" You reply, resisting the urge to glance over your shoulder.

The door opens soundlessly. As soon as it's open far enough for you to enter, you scramble inside, jump up, and wrap your arms around Sans' neck with a small squeal. He catches you by the waist on reflex, letting out a surprised laugh. He's so large compared to you that you always feel like a spider-monkey when he does this.

"Jesus God shit piss fuck," You breathe out.

He laughs again. "You okay?"

"It's freaking creepy out there." You explain.

You beg him to close the door, quietly pleased when he does so while still supporting you with one arm.

"Please don't put me down," you say, when it seems like he's going to bend down and plant you on the floor. You're still a little spooked. Sans shrugs and carries you over to his desk, his pace as unhurried as ever. You curiously peer around his little space. File cabinets line the walls. There are papers  _everywhere._ The best way to describe it would be an organized mess. You glimpse a framed picture of you on the wall, next to a picture of Papyrus, and you bashfully look away, hoping that Sans doesn't notice the grin that crosses your face.

"What's up?" Sans asks.

"I just wanted to see you! Alphys gave me the key." You answer him, smilingly adjusting the rumpled collar of the flannel shirt he's wearing.

"Oh? Cool. Babe, welcome to my humble lab a- _bone_." He answers. His grin looks a little sheepish.

Sans plops down in the big leather chair behind his desk. You squirm in his lap, moving until you face his desk. You stare at the papers on it with no small amount of confusion. You can't understand a word that's been written on any of them. Do monsters have their own language, or is this just a code?

"Babe... Why are you wearing my jacket?" Sans asks, after a beat of silence. There's a familiar tone to his voice that spreads warmth in your belly. He cradles your hips, effectively drawing your attention away from the strange markings on his papers.

You clear your throat and begin shuffling the papers before you.

"Oh, that? I was just thinking about how nice it would be if you were with me. I put it on to wear around the house." You say. You begin to stack the papers neatly, placing them in a far corner, where they'll be out of the way.

"And this skirt?" Sans asks, running a hand up your thigh. You shiver at the press of his bones against your skin.

"When Alphys gave me the keys, I was in a rush to get here and see you, so I just threw on whatever." Is your innocent reply.

His hand slides over to your inner-thigh, travelling upwards. You gasp when his hand reaches its destination.

"You were in such a rush that you forgot your underwear? You must have been real excited to come here." Sans says, conversationally.

The tips of his fingers catch on the nub of your clit, rubbing over the hood in slow circles. A low, warm heat blossoms between your legs, radiating outwards until you release a sobbing breath to calm yourself. It's hard to resist the urge to roll your hips into his touch. 

"You have no idea." You say, voice pitching high on the last word. You bat Sans' and away, and stand. You turn to face him, lean in, and curl your hands around his shoulders. You push forward until he obligingly helps you move his chair away from his desk. His eyes widen when you get down on your knees.

"You, uh. You really don't have to do that—" Sans is saying, as you undo the fly of his faded jeans. You ignore him, and pull them down to reveal his pelvic cradle.

"Hush. I've told you before, and I'll say it again. I want to." You say.

You can almost feel another half-hearted protest rising in his chest. You lean in and trail your tongue along his pubic bone. The reaction is almost instantaneous. He moans lowly, releasing full-body shudder. The back of his skull hits the wall behind him with an audible thunk. You reach up to unbutton his shirt, just enough to allow you to reach your hand underneath to run your hands along the length of his ribs, which are thicker than any human's. You've always been just as fascinated with his body as he is with yours, from the odd warmth that radiates off of every inch of him, to the areas that made him shudder.

You curl one hand around his coccyx, your thumb rubbing slow circles into the ridges of the rigid bone. It feels heavy in your hand, comfortingly solid,  _real._ Sans lets out a rumbling growl, reaching down to cup your cheek. You turn your head to kiss his palm, and then, locking gazes with him, you sucking his fingers into your mouth. You can taste your own arousal on each digit. 

"Shit, babe. Fuck," Sans groans, sliding his other hand into your hair, the tips of his phalanges scratching warmly against your scalp. His sockets fall closed. There's sweat beading on his scalp—yet another strange and oddly endearing part of him that still amazes you.

The thing is, you really like doing this. You like making him feel good. You used to roll your eyes at the people who say sex with someone you love is better, but you're starting to understand just what they mean. The thrill of knowing that you're able to reduce him to an incoherent mess with just your fingers and tongue is a high that's both selfless and entirely selfish. You rub your thighs together to ease the ache radiating from between them.

"That's what I'm here for, bone-boy." You answer, grinning at the husky laugh it gets you. Caught up in the moment, you pull your hands from his body to cradle the back of his skull, pulling him down so that you're face-to-face. You plant a soft kiss on his cheekbone. He sighs and leans into you, and when you part, you gaze quietly at his face, grateful for the chance to calm yourself. Feeling just a bit warm, you unzip his jacket until your torso is bared. The conditioned air feels good against your skin.

When Sans opens sockets, his left eye is glowing with a familiar blue light. You glance down, and he says, "Well, I was feeling kind of  _bonely_ working through all this data by myself."

"You'll stay that way, if you keep going down that road," You warn him, though you're biting your cheeks to hide your smile. 

"You mean you weren't thinking about this  _boner,"_ He points down to where his blue cock is peeking up at you from his jeans, "while you were at home alone?"

You retaliate by curling your hand around his shaft and giving it a quick tug. He curses, and his head hits the wall behind him yet again.

"Look at me." You command him, and when he does, look into the bright pinpricks of his eyes and kiss his shaft. You slide your tongue from the base of his cock to its head. Your curl your hand back around his length as you flick your tongue over the slit. Your eyes fall closed when his fingers delve back into your hair, a comforting weight.

"Feels good, babe." Sans tells you, letting out an appreciative moan when you pop the head into your mouth and swirl your tongue around it. You take as much of him into your mouth as you can into your mouth, pumping what you can't reach with your hand. You shift, widening your stance, and with your free hand, you thrust two fingers inside yourself. Enough to take off the edge of your mounting arousal, but not enough to botch your concentration.

You bob your head slowly, relishing in his unique taste, light musk and a hint of sweetness. Sans doesn't let you do this enough. He likes being in control, and is a fairly indulgent lover, but you suspect it stems from a private fear a vulnerability. You've gotten hints of it in the past. You enjoy this, though—the slight pain of the hard floor beneath your knees, the heaviness of his cock in your mouth, the taste of him, the sounds he releases every time you press your tongue flat against him and give a little twist on the upwards stroke of your hand.

You press your thumb against your aching clit and moan at the relief the momentary friction gives you. Sans gives a surprised grunt, thrusting up into your mouth. The head of his cock touches the back of your throat and you move your head back, your eyes watering.

"Shit. Sorry, sorry." He mumbles, tracing your wet, swollen lips with his thumb. 

"It's okay," you soothe him, clearing your throat.

You place both of your hands on his thigh bones, and smile up at him.

"Like that, huh?"

You take him in your mouth again before he can reply, humming lightly as you bob your head along his length. 

"Fuck, babe. You feel so good." Sans groans. You can feel his eyes on you as you move, and it urges you to go faster, suddenly greedy for more.

"I'm gonna cum, baby. Just like that. Shit," Sans hisses.

You pull up until just the tip of him is in your mouth, and you suck intently, tugging on him until he lets out a hitching gasp and cums in thick spurts. You swallow as much as you can, rolling the thickness over your tongue. When you pull away from him, a thin blue stream connects your lips to his head. You lick your lips and it snaps. Sans stares down at you in wide-eyed silence, and you blushingly wipe your mouth on the sleeve of his jacket to escape his gaze.

"Oh my fucking fuck," Sans blurts out. 

You giggle, and it trails off into a small shriek when he reaches down and hauls you up onto his desk by your waist.

"I'm guessing you liked that?" You ask, breathlessly. 

He presses his forehead into your belly and gives you an emphatic  _Yes._ You share a laugh.

He pulls you towards the edge of his desk. Its surface is uncomfortably hard beneath your bare skin, but you don't really mind. You watch with bated breath as he moves his head lower, and lower. He grasps the hem of your short skirt and moves it up your thighs until you're bared to him.

"Where'd you get this, anyway?" He lets out a huffing laugh, and you shudder at the way his hot breath puffs against your slit.

"Would you believe me if I told you Mettaton helped me pick it out like two months ago?"

He freezes, and you let out a loud laugh.

"Let's, uh. Not think about that." He finally says, curling his hands under your thighs and pulling you forward.

He hooks your knees over his shoulders. You're spread wide by the breadth of them, leaving you feeling entirely naked despite the jacket clinging to your sweaty skin and the skirt splayed against your heaving stomach.

"Damn, that made you really wet." Sans announces, a pleased grin crossing his face.

He leans in, pressing his face into your inner thigh. A high-pitched moan falls from your lips as his teeth sink into the sensitive skin, just enough for you to really feel the sweet ache that follows. His tongue darts out and laves over the shallow bite-marks left behind. You rest a hand on his head, and lay your head back against his desk as he switches to your other thigh and repeats the process.

"Stop teasing me," you gasp, frustratedly rolling your hips.

Sans' hands are tight on the globes of your ass, holding you up as he slides his tongue up your aching slit. He moans at your taste, easily riding out the sharp, involuntary jerk of your hips. The tip of his tongue circles your clit for one quick, delicious second before darting back down your slit, teasingly flicking at your swollen inner lips.

You let out a sobbing cry of his name, tossing your head to the side. You reach up to twist the hardened nub of your nipples, squeezing at your breasts before moving to cradle his head once again. He presses his tongue flat against your clit, and you eagerly ride his face like this, with his hands locked tightly against your thighs to hold you up.

Just when you're sure you can feel your orgasm sneaking up on you, starting as an ache at your empty core and building as a fire in your belly, Sans moves his face from between your thighs. You give a frustrated cry, your hips bucking against air.

"Sans, what—"

He shifts, resting your bottom against his desk once again. 

"Chill, 's okay, babe." He rumbles.

He hooks your knees over his shoulders once again, but this time he inserts two fingers inside of you you. You're soaking, so he meets no resistance, sliding in to the last knuckle in one smooth movement. He crooks his fingers up, and you spasm, riding the movement with a warbled cry. He leans in and licks at where you are stretched around his fingers, then he slides another finger inside. For a second, you feel a light burn. Then his tongue catches on your clit, not once stopping in its intent circling movements against the hardened nub, and the slight pain is completely forgotten.

You're an incoherent mess. Your hands are everywhere—on his head, pulling at his flannel, running over your breasts and into your own hair. 

"Don't stop, don't stop," you cry out, as your walls start to flutter around Sans' fingers.

He pauses to coax one of your legs off of his shoulders, and presses the flat of his palm against your mons. He crooks his fingers inside you again. You tense up, and then buck wildly in his hold, cumming with a silent scream. 

You have to lean up on your elbows and push at his shoulder to get him to move, once the touch of his tongue starts to hurt just a bit more each time it passes over your clit. He slides his fingers out of you, one-by-one. He licks your slick off of his fingers, not once looking away from your face.

You fall back against his desk and throw an arm over your eyes, breathing deeply through your nose. Sans cradles your hip with one hand, the other rubbing circles on your belly before grabbing your free hand and rubbing his thumb over the skin of your wrist. Feeling for your pulse is a habit of his.

"I'm guessing you liked that?" He teasingly asks, throwing your words back at you.

Your response is unintelligible each time you try to form words, so you settle for nodding and stroking the curve of his skull. He chuckles and presses his face into your stomach. The two of you fall into a comfortable silence. The weight of him pressed into your lower body makes you feel safe and protected. 

"Take me home?" You ask, once you've caught your breath.

You pause, and add, "I know you can teleport."

He says, "Well, shit."

You laugh, and his head bobs with the movement. He rests his chin on your stomach and looks up at you, looking more than a little guilty.

"You've been cooped in here for a bit. You should rest. If you stay at home with me, you can tell me the whole story." You offer.

His wary expression softens. "Sounds like a plan."

He helps you up, and then smiles down at you.

"I know a shortcut." He begins.

You snicker. "That sounds rehearsed!"

"It was!" He laughs. "I wanted to look all cool. But, uh, yeah. Hold on tight."

The best way to describe Sans' shortcut would be backwards vertigo. It makes no sense, and that's the only explanation that makes sense. You realize there's probably a lot about Sans that you don't know.

When the two of you land in the living room of your house, he bends down, locks an arm under your weak knees and aching back, and carries you to the bathroom to clean up. You curl your arms around his shoulders and realize something else: you don't really care that much. He'll tell you more when he's ready. For now, you're glad that he'll have you.

**Author's Note:**

> Edit: my buddy Tan did some [cool](http://tansans.tumblr.com/post/137986319974/im-obsessed-its-fine-kinda-inspired-by) [art](http://tansans.tumblr.com/post/138060480834/incoherent-screaming-okay-so-freidacays) inspired by this fic! The second picture is NSFW.
> 
> Find me at [Tumblr](http://freidacay.tumblr.com) so we can shout at each other about skeletons and Undertale.
> 
> Hope you enjoyed this! Thanks for being patient with me. Getting back into writing has been a real experience. I have to find a good balance, I think. Please let me know how I'm doing! 
> 
> I love ScienceBros! Sans and Alphys. This was inspired by the stray thought Sans could sometimes be so In The Zone that he'd forget about everything else and eventually crash. But I think you prevented that this time ( ͡° ͜ʖ ͡°)


End file.
